Japanese Laid-Open Patent Publication No. 2003-20930 describes a technique for burning particulate matter (PM) accumulated in a filter, which is arranged in an exhaust passage of a diesel engine, when the amount of PM accumulated in the filter exceeds a predetermined amount. The PM accumulated in the filter is burned by heating the filter and intermittently adjusting the air-fuel ratio to the lean side. In this prior art, the amount of PM accumulated in the filter is estimated by cyclically adding the amount of PM emitted from the engine and the amount of PM oxidized in the filter based on the driving state of the engine.
When the engine driving state is changing, the actual PM emission amount and the PM oxidation amount may not be the same and differ from each other. In particular, the estimated PM accumulation amount may be less than the actual PM accumulation amount. When the actual accumulation amount is greater than the estimated accumulation amount, the elimination of the accumulated PM may be insufficient. If such insufficient elimination is repeated, an excessively large amount of PM may be accumulated. In such a case, a greater amount of PM than intended may be rapidly burned. As a result, the filter would become overheated. This would cause thermal deterioration of the filter.